The effect of cobalt/copper ions on the structural, thermal, optical, and emission properties of erbium zinc lead borate glasses

A host glass network of 70B2O3–10Pb3O4–18ZnO–2Er2O3 (ErCoCu1) was proposed and the impact of 1 mol% of Co or/and Cu ions on its structural, thermal, optical, and green emission properties was studied extensively. The X-ray diffraction spectra confirmed the amorphous structure of the produced glasses. Density and density-based parameters behavior showed that the Co or/and Cu ions fill the interstitial positions of the proposed ErCoCu1 network, causing its compactness. Both ATR-FTIR and Raman Spectra affirmed the formation of the fundamental structural units of the borate network, B–O–B linkage, BO3, and BO4. Additionally, the penetration of Co or/and Cu ions inside the ErCoCu1 converts the tetrahedral BO4 units to triangle BO3 causing its richness by non-bridging oxygens. The addition of Co or/and Cu reduces the glass transition temperature as a result of the conversion of the BO4 to BO3 units. Optical absorption spectra for the host glass ErCoCu1 showed many of the distinguished absorption bands of the Er3+ ion. Penetration of Co ion generates two broadbands referring to the presence of Co2+ ions in both tetrahedral and octahedral coordination and Co3+ ions in the tetrahedral coordination. In the Cu-doped glasses, the characteristic absorption bands of Cu2+ and Cu+ were observed. A green emission was generated from the ErCoCu1 glass under 380 nm excitation wavelength. Moreover, no significant effect of Co or/and Cu on the emission spectra was recorded. The considered glasses had appropriate properties qualifying them for optoelectronics and nonlinear optics applications.

The multiple oxidation states of the transition metal ions TMIs rich the glasses networks by many optical, electrical, and magnetic properties [1][2][3] . Optically, the TMIs give various specular colors to the glass networks, making them have a high optical absorption ability in the different regions of the electromagnetic spectrum such as UV, visible, and IR regions [4][5][6] . From the photoluminescence point of view, TMIs generate broad emission bands that have an adjustable wavelength and appropriate quantum yield 7,8 . Electrically and magnetically, the multiple oxidation states of the TMIs bring substantial modifications in the glass networks structural units by influencing the charge degree of freedom and spin, which in turn directly affect the conduction process and the electrical and magnetic nature of the glass network 9,10 . Hence, glass-containing TMIs have significant applications in photonics, electronic, optoelectronics, and magnetic domains such as light emitting diodes, optical filters, solid-state lasers, memory-switching electronics, superionic batteries, catalysis, smart electronic devices, and magnetic information storage [11][12][13] . Cobalt (Co 2+ /Co 3+ ) and copper (Cu + /Cu 2+ ) ions are of the most distinctive transition metal ions in enhancing the properties of various glass networks. The formation of the mixed valence states of cobalt ions (Co 2+ /Co 3+ ) in octahedral (oh) and tetrahedral (Td) geometric forms inside the glass network makes it a favorable material in solar selective absorbers, fuel cells, visible and NIR-lasing materials, supercapacitors, gas sensors, and lithium-ion batteries. Cobalt imparts a blue or pink color to the glass depending on the Co 2+ ion geometrical shape coordination (tetrahedral or octahedral) [14][15][16] . Adding Cu ions to glasses networks generates two valence states, Cu + and Cu 2+ , during the preparation process under normal conditions. Cu ions usually add a blue or green color to the glass network. In general, the formation of divalent copper ion Cu 2+ can be determined based on the formed color in the glass. In addition, the Cu 2+ ion form a broad absorption band in the visible-near infrared range that usually arises due to the octahedral coordination of Cu 2+ , while the cuprous (monovalent copper) ion Cu + has a distinct absorption band in the UV region. These absorption bands are usually Measurements and theoretical aspects. Structural properties. First, the formation of the amorphous phase of the prepared materials was tested by X-ray diffraction (XRD) patterns. A Philips X-ray diffractometer using a monochromatic Cu-Kα radiation of wavelength 1.54056 Å was used to record the X-ray diffraction spectra. Density and density-based parameters effectively explore the impact of the additives on the physical properties of glass networks, so the density ρ was measured using Archimedes' principle according to Eq. 1 3,9,23,24 then density-based parameters (molar volume V m , mean boron-boron separation d B−B , oxygen packing density OPD , and packing density PD ) were deduced using the Eqs. 2 3,9,25-27 , 3 12,28 , 4 12,26 , and 5 26 . Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), (Alpha-Bruker) was used to study the functional groups of the studied glasses in the spectral range of 400-4000 cm −1 . Gaussian deconvolution was conducted to pry the origin of the formed broadbands in the ATR-FTIR spectra. The formed tetrahedral BO 4 ( N 4 ) and trigonal BO 3 ( N 3 ) units ratios inside the considered glass network were calculated using Eqs. 6 and 7 26 .
Raman spectra were recorded for the considered glasses by SENTERRA Dispersive Raman Microscope (Bruker) equipped with a diode Nd:YAG laser in the spectral region of 500-4000 cm −1 . A narrow-spectrum 532 nm laser excitation system was used to illuminate the produced glasses. Also as the same as of ATR-FTIR, a Gaussian deconvolution to resolve the formed broadbands in the Raman spectra was carried out.
Thermal properties. The glass transition temperature of the considered glasses was deduced through a Differential Scanning Calorimeter test using a Mettler-Toledo Instruments, at a 5 °C/min heating rate.
Optical properties. UV-VIS-NIR absorption spectra were measured using a JASCO V-670 UV/Vis spectrophotometer in the spectral region of 200-1600 nm with a 2 nm resolution. Smooth and flat glass samples with dimensions 2 cm 2 and thicknesses 1.1-1.4 mm were used in the optical spectra measurements. To determine the existence of various oxidation states of both Co and Cu ions in different geometrical shapes (octahedral and tetrahedral coordination), a deconvolution of the optical absorption spectra was conducted. Based on the obtained absorption bands of Co ions, the ligand field parameters around Co ion; crystal field splitting coefficient 10Dq, Racah parameters B&C, and nephelauxetic ratio β were estimated through the following relations 24,25,29. (2) www.nature.com/scientificreports/ where B freeion of Co is 1120 and ν 2 and ν 3 pointed to the bands corresponding to the energy of electronic transitions in tetrahedral Co 2+ ions, ν 2 is the electronic transition corresponding to the visible region and ν 3 is that to the NIR region. Based on the optical absorption spectra, the optical band gap of the considered glasses was deduced according to Mott-Davis theory using the following equation 9,10,14,24 . where α , hν , B , E opt are the absorption coefficient, the incident photon energy, the band tailing parameter, and the optical band gap energy respectively. The n signifies the kind of the occurred electronic transition and take values 1/3 & 1/2 for direct transitions and 2 & 3 for indirect ones. Generally, for glassy materials, the indirect allowed transition is the dominant; hence, the E opt values were deduced for the n = 2 only. The value of E opt estimated through plot the Tauc relation between hν and (αhν) 0.5 and by extrapolating the linear part of the curve at (αhν) 0.5 = 0.
To measure the disorder degree in the considered glasses, the band tail, which known as Urbach energy E was estimated through the following equation 9,10,14,24. where β is a constant.
The value of Urbach energy determine through the inclination of the linear regions of lnα − hν relation and taking its reciprocal.
The steepness coefficient S, which measures the width of the band-tail in the main gap, was calculated using the formula 28,29. where β , K B , and T are the exponential constant, Boltzmann constant, and room temperature.
Based on some mathematical contractions, the values of the steepness coefficient can be obtained through a simple relationship that links the steepness coefficient with Urbach energy, which is Depending on the band gap values, the conducting behavior of the solid (conductor, semiconductor, or insulator) can be characterized. Metallization criterion (M), which is calculated from Eq. 16, is used to determine the conducting behavior of the solid precisely. According to Herzfeld's theory of metallization of the condensed matter, the M value may be approached to zero, close to one, or stands between them, which reflects the conductive, insulating, or semiconducting nature of the solid respectively 28,29 .
Photoluminescence. Agilent-Cary eclipse fluorescence spectrophotometer equipped with Xe-lamp was used to record the emission spectra in the spectral region of 380-800 nm. The considered glasses were excited using 380 nm excitation wavelength.

Results and discussion
Structural properties. XRD. The X-ray diffraction spectra, which are displayed in Fig. 1 signify the amorphicity phase formation of the prepared materials, where, as observed, a broad halo in the spectral range of 22-31° has appeared. It is known that the borate glass forms a broad halo in the spectral range 2θ = 25−30°3 0,31 . So, the observed widening in the formed X-ray diffraction peak for the proposed host network, which extended from 22° to 31° arose as a result of the role of Pb 3 O 4 , ZnO, and Er 2 O 3 in the network formation. No significant shift in the formed broad halo was observed with the inclusion of Co or/and Cu ions, due to they are being added in small concentrations.
Density and density-based parameters. Table 2 displays the variation of density and density-based parameters, molar volume V m , mean boron-boron separation d B−B , oxygen packing density OPD , and packing density PD , as a result of the penetration of Co or/and Cu ions. First and in general, a weak augmentation in the density and reduction in molar volume with the inlying of Co or/and Cu ions compared to the host glass ErCoCu1 was observed as listed in Table 2. The density augmentation resulted from the fact that the two suggested transition metal ions, Co and Cu, were added to the host network as a doped and not as a substitution. The observed growth in the density and the resulting reduction in molar volume arose from the filling of the interstitial spaces of the glass network by the Co or/and Cu ions, causing shrinking of the interfacial distances and tighter packing. The shrinking of the mean boron-boron separation confirmed the role of the Co or/and Cu ions in filling the interstitial spaces within the studied network, as their penetration into these voids causes them to rival the boron atoms causing their displacement towards each other. As a result of filling the interstitial spaces with Co or/and Cu ions and reducing the mean boron-boron separation, the considered glass network was tightened, which was www.nature.com/scientificreports/ evident in the increase in OPD and PD as listed in Table 2. On the other hand, the glass containing higher Cu ions concentrations (ErCoCu3 and ErCoCu6) than Co ions (ErCoCu2 and ErCoCu5) showed a higher density due to the higher molecular mass of Cu (63.5 gm/mol) compared to Co (58.933 g/mol). On the other hand, due to the ionic radius of Cu (0.073 nm) is higher than that of Co (0.072 nm), the range of its filling to the network voids had expanded causing a higher reduction in the molar volume for the glass containing higher concentrations of Cu compared to that containing higher concentrations of Co.    [28][29][30][31][32][33] . Co ions inclusion (ErCoCu2 glass) increased the intensity and relative area of this band, while an opposite trend was observed with Cu ions inclusion (ErCoCu3). This behavior reflects the network richness of the glass containing 1 mol% of Co ions by B-O-B bending vibration compared to that containing 1 mol% of Cu ions. In the glass containing the mixture of Co and Cu ions (ErCoCu4, ErCoCu5, and ErCoCu6), it was observed that the role of Co ions in enhancing the intensity and relative area of this band is dominant. Finally, a shift towards the higher energy occurred in the glass containing 0.5 mol% of Co and 0.5 mol% of Cu while the relative area increased. The deconvolution of BO 4 unit broadband, which appeared here in the spectral region 770-1120 cm − Table 2.
Raman spectroscopy. Figure 3 shows the recorded Raman spectra for the considered glasses and their deconvolutions (host glass ErCoCu1 as an example). In Fig. 3a, the observed band in the low energy region 50-125 cm −1 arose due to the vibrational modes of BO 3 54,55 , which is clearly shown in Table 2 in the behavior of N 3 and N 4 . The occurred contraction in the T g was entirely in tune with the concentration of BO 3 within the glass network, where the glasses containing higher concentrations of BO 3 showed a lower glass transition temperature than those containing lower concentrations.
Optical properties. In the host glass ErCoCu1, a band at 308 nm and ten of the characteristic absorption bands of Er 3+ ion at 346, 384, 426, 522, 546, 650, 800, 980, 1512, and 1554 nm have appeared as displayed in Fig. 5a. The absorption band located at 308 nm arose due to of the electron transition in the non-bridging oxygen and/or the electron transition in the divalent Pb 2+ ions 56,57 Fig. 5a, two broadbands appeared in the spectral region of 400-650 nm and 1250-1500 nm in addition to the presence of the bands at 1512 and 1554 nm, which results from 4 I 15/2 → 4 I 13/2 transition as mentioned previously 59,61 . The deconvolution of the appeared two broadbands generated ten bands at 452, 472, 522, 566, 610 1272, 1310, 1346, 1432, and 1448 nm as displayed in Fig. 5b. The two bands at 452 and 522 nm arose due to the transition 4 I 11/2 → 4 F 5/2 and 4 I 15/2 → 2 H 11/2 in Er 3+ ions 59,60 . The located bands at 472 and 566 nm arose due to the transitions 4 T 1g (F)→ 2 T 2g (F) in the octahedral Co 2+ and spin-forbidden transitions 4 A 2g ( 4 F)→ 4 T 1g ( 4 P) in the tetrahedral Co 2+ , while that at 610 nm arose due to 5 T 2g → 5 E g transition in the octahedral Co 3+32,62-65 . The ground state 4 F of tetrahedral field of Co 2+ splits to 4 A 2 , 4 T 2 , and 4 T 1 , while 2 G splits to 2 A 1g (G), 2 T 1g (G), 2 T 2g (G), and 2 E g (G) levels 66,67 . Moreover, 4 P only transforms to 4 T 1 ( 4 P) level. So, the bands centered at 1272, 1310, 1432, and 1448 nm arose as a result to the transition between the ground state Ŵ 8 ( 4 A 2 , 4 F) and the excited states Ŵ 6 , Ŵ 7+8 , Ŵ 7 andŴ 8 of 4 T 1 ( 4 F) [66][67][68][69][70] due to the first and secondorder spin-orbit coupling effects. The absence of the three bands, which appeared at 346, 384, and 426 nm in ErCoCu1 in this sample, maybe due to their overlap with the two bands appearing at 452 and 472 nm. In Cu doped glass ErCuCo3, in addition to the bands located at 288, 314, 354, 384, 414, 492, 522, 548, 984, 1540, and 1584; a broadband appeared in the region of 740-1140 nm and deconvoluted to 838, 908, 984, and 1058 nm as shown in Fig. 5c. The copper ion usually exists in the two most stable valence states, Cu + and Cu 2+ . The fulfilled 3d 10 configuration cuprous ion Cu + shows an absorption band in the UV-blue region due to the 3d 10 → 3d 9 4s 1 transition, therefore the centered band at 288 nm is refers to the existence of Cu + ion in the ErCoCu3 glass 71,72 . For copper Cu 2+ , which is usually present in octahedral coordination, during the melting process; a splitting in the d-orbitals into the doubly degenerate 2 E g (higher energy) and the triply degenerate 2 T 2g (lower energy) occurs. Moreover and due to the tetrahedral distortion, 2 Eg splits to 2 B 2g (dx 2 − y 2 ) and 2 A 2g (dz 2 ), while 2 T 2g splits to 2 B 2g (d xy ) and 2 E g (d xz , d yz ) through the Jahn-teller effect, which causes a breadth in the shape of the formed peak. Hence, the located bands at 838 (11,933 cm −1 ), 908 (11,013 cm −1 ), and 1058 nm (9452 cm −1 ) are assigned to 2 E g → 2 B 1g , 2 B 2g → 2 B 1g , and 2 A 1g → 2 B 1g transitions respectively [73][74][75] . The others appeared bands in the ErCoCu3 glass at 314, 354, 384, 414, 492, 522, 548, 984, and 1540 & 1584 nm are assigned to the transition in Er 3+ ion from the ground state 4 I 15/2 to the excited states 2 D 3/2 , 2 K 15/2 , 4 G 11/2 , 4 F 3/2 , 4 F 7/2 , 2 H 11/2 , 4 S 3/2 , 4 I 11/2 , and 4 I 13/2 respectively 58,59,76,77 . In the ErCoCu4, ErCoCu5, and ErCoCu6 glasses, which contain a mixture of Co and Cu ions, the same broadbands are formed as shown in Fig. 5a in the spectral ranges of 400-650, 740-1140, and 1250-1500 nm signifying the presence of octahedral (Oh) and tetrahedral (Td) of Co 2+ , tetrahedral of Cu 2+ , and tetrahedral Co 3+ respectively. It is also worth mentioning that, the characteristic band of Cu + ions is continued to exist in these glasses.    www.nature.com/scientificreports/ The deduced values of the crystal field splitting coefficient 10Dq, Racah parameters B&C, and nephelauxetic ratio β are listed in Table 3. It was found that the Racah parameters, which generally use to measure the Coulomb repulsion within the d-shell decrease with the increase of Cu ions and a decrease of Co ions concentrations. The observed reduction in Rach parameters refers to the covalency nature of the bonds between Co ions and ligands. On the other hand, the observed reduction in the 10Dq signifies that the Co 2+ ions have a strong localization in the considered glass network. Nephelauxetic ratio β , which measures the stability of ions (Co 2+ ions here) complexes and their interaction mechanisms, was found to be increased with Co ions augmentation and Cu ions reduction. The reported growth in Nephelauxetic ratio values indicated the augmentation in the stability of Co 2+ ions in the considered glasses. The 10Dq/B ratio, which measures the interaction strength, showed that the crystal field sites of the considered glasses are within the strong interaction regime and tend toward a strong crystal field.
Tauc relationship between hν and (αhν) 0.5 was plotted as shown in Fig. 6a (glass ErCoCu1 as an example) to deduce the values of optical band gap for the considered glass. On the other hand, to estimate the Urbach energy values, a relationship between hν and lnα was plotted as shown in Fig. 6b (glass ErCoCu1 as an example). Generally, a reduction in optical band gap and the augmentation in Urbach energy with the addition of Co or/ and Cu ions compared to the host glass was observed as listed in Table 3.
There are two main reasons for the occurred reduction in the optical band gap and augmentation in Urbach energy. The general one is the formed non-bridged oxygen NBOs in the energy gap near valence and conduction edges. The NBOs behave like donor centers inside the band gap, which cause its shrinking. Also, the linked excited electrons by the non-bridging oxygen are less tight than those linked by the bridging oxygen, which in turn decreases the optical band gap. The specific one is that (i) the gradual augmentation of Co ions in the octahedral position formed a large number of donor centers leading to overlapping between the trapped excited states of localized electrons on Co 2+ sites and the unfilled 3d states on the neighboring Co 3+ sites. Hence, a wide extension of the impurity or polaron band in the band gap takes place leading to a reduction in the optical band gap 16 . (ii) Cu ions like Co create a large number of donor centers. In Cu ions, the trapped excited states of localized electrons are on Cu + sites and overlap with the unfilled 3d states on the neighboring Cu 2+ sites 78,79 . The width of the formed tails due to the agglomerations of NBO, Co, and Cu in the main band gap and the augmentation of the disorder were clearly confirmed by the obtained values of steepness coefficient S. Inclusion of Co or/and Cu reduce the value of steepness coefficient reflecting the shrinking of the edge broadening confirming the disorder augmentation and band gap reduction. The optical band gap values of the glasses containing Co or/ and Cu ranged between 1.62 and 2.23 eV, which means that they have a semiconducting nature. The obtained values of the metallization criterion as listed in Table 3 confirmed the semiconducting nature of the considered glasses. The small values of the metalization criterion of the considered glasses refer that the width of both valence and conduction bands becomes large, generating a narrow band gap and enhancing the tendency of the glass for the semiconducting nature. Moreover, the values of the metallization criterion, which ranged from 0.285 to 0.348 indicated that the considered glasses have non-linear refractive indices, which means those glasses have non-linear optical properties 80 . The non-linear refractive index of the considered glasses was computed using the equation 81,82. where B = 1.26 × 10 −9 eV 4 .
Generally, compared to the host glass ErCoCu1, the glasses containing Co or/and Cu possessed high nonlinear refractive indices as listed in Table 3, which arose as a result of the increased disorder within the glass network with the penetration of Co or/and Cu ions as confirmed by Urbach energy. The growth of the non-linear refractive index of the ErCoCu1 with the introduction of Co or/and Cu confirmed the enhancement of the nonlinear optical properties of the produced glasses. www.nature.com/scientificreports/ Emission analysis. Emission spectra and electronic transition. Through excitation of ErCoCu1 glass using the 380 nm, three emission bands; one in the blue light region at 488 nm and two in the green region at 520 and 533 nm are generated as shown in Fig. 7a. Under 380 nm excitations wavelength, a quite well population of 4 G 11/2 occurred from the ground state absorption (GSA) 4 I 15/2 as shown in Fig. 7b. Non-radioactive decay via transition to the 4 F 7/2 excited state occurred, followed by radiative decay generating three lines of blue-green emission at 488, 520, and 533 nm. Sometimes, multi-photon electron relaxation from the excited state 4 S 3/2 to 4 F 9/2 takes place followed by the 4 F 9/2 → 4 I 15/2 transition, leading to the emission of a red wavelength line, which is not observed here. Not noticing a red emission here means that the 4 S 3/2 → 4 F 9/2 relaxation is not occurring. No variation in the observed emission bands, in position or intensity, was observed with the inclusion of Co or/and Cu. The absence of any change in the emission spectra means that the used wavelength cannot cause any excitation in the Co or Cu ions. Moreover, no energy transfer occurred between Co and Er 3+ or Cu and Er 3+ .
Green light emission and colorimetric analysis. The appropriate combination of the emitted blue and green emissions generates a green light as shown in the CIE 1931 chromaticity diagram in Fig. 7c. The CIE coordinates of the considered glasses were located at x s = 0.192 and y s = 0.517 , which is close to the green light of the National Television System Committee (NTSC). The absence of the effect of Co or Cu ions on the intensity and position of the emission bands was clearly reflected on the coordinates of the generated green color, as they all appeared in the same position without any significant change. Xiaojian Pan et al. obtained almost the same coordinates for the emitted green light from CaNb2O6:Tb 3+ phosphor under 260 nm excitation 83